This invention relates to a wet multiple disc clutch system in a transmission particularly adapted for mounting on an industrial or construction vehicle such as a forklift, shovel-loader or the like.
The clutch system of this type comprises a drive plate into which the engine power is fed, a driven plate connected to the output shaft and a piston which presses the two plates together and is operated hydraulically by a controlled oil pressure supply. In the absence of a cushioning mechanism, however, so-called "select shock" may take place, in which the vehicle starts abruptly at the moment of clutch engagement (i.e., the moment the clutch cylinder is filled with oil). That is, as oil is being supplies to the cylinder, there is a pressure difference between the pressure controller and the cylinder due to the resistance of the oil passage to the flow of oil. However, the moment the clutch cylinder becomes full the flow stops, and therefore the pressure difference disappears; thus the pressure in the cylinder rises abruptly and as a result the clutch fails to be engaged smoothly. This select shock may be relatively slight with heavyweight or loaded vehicles, but with unloaded or light class vehicle weighing less than a ton, the shock is strong because the latter can start with a low torque. The pressure relationship can be written: EQU Pm=Pc+pa,
where Pm is the supply pressure at the pressure controller, Pc is the pressure in the clutch cylinder and Pa is the pressure drop caused by the resistance in the passage from the pressure controller to the clutch. Pm is never less than Pc, but Pc equals Pm abruptly at the moment of clutch engagement (the moment the clutch cylinder is filled with oil and the flow in the passage stops). Consequently, as long as Pa is greater than zero, the abrupt rise of Pc (the pressure in the clutch cylinder) at the instant t1, at which the clutch cylinder is filled with oil cannot be substantially avoided, as shown in FIG. 7(b). A shock torque, therefore, is produced as shown in FIG. 7(a), and the value "a" off peak torque also is high. This problem can be solved if Pa is made zero, that is, the pressure drop is eliminated. For this, however, the distance between the modulator valve and the clutch must be minimized, and the oil passage therebetween must be diametrically enlarged. Furthermore, the passage would need to have no sharp bends or joints which offer resistance. In practice, such a design is extremely difficult to implement due to the limitations of the transmission layout and the size requirement or manufacturing conditions.
It has been proposed to form the steel drive plate 24 in a conical shape as illustrated in FIG. 1, to obtain a cushioning action from the plate itself, but in that case the facing of the driven plate 22 angularly collides with the steel drive plate 24, temporatily creating abnormally high pressures on the facing surface. This could induce burning or abnormal wear, particularly with the recently appearing paper facing of driven plate 22 (graphite-containing paper or the like), and hence the conical steel drive plate 24 is regarded as impractical. There has been another proposal in which the conical plate 28a is seated on a stepped retainer plate 28 as shown in FIG. 2. This also has the drawback in that, when the clutch facing 22 wears away, the space between the clutches is enlarged also due to the fatigue of the conical plate 28a, to catch the conical plate 28a which has come out of the stepped portion of the retainer plate 28, failing to achieve the intended effect. Furthermore, in the design, a steel drive plate must be additionally inserted between the conical plate 28a and the driven plate 22 to prevent the wear of the latter, which increases production costs.
As a solution to the above problem, it has been proposed to insert a conical plate 110 between the drive plate 24 and piston 14 as shown in FIG. 3. However, in the proposed design the conical plate 110 rotates freely and due to its vibration, wears away the engaging groove 13b of clutch drum 13. The conical plate 110 is also apt to be caught by the worn out portion which hampers the proper clutch action. Particularly, with a forklift, shifting between the forward and reverse gears is frequent and hence, the wear caused by the oscillation of the conical plate is very appreciable, and noise is often caused.